Heat recoverable articles, i.e. those which have been deformed from a first heat-stable configuration to a second heat-unstable configuration and which are capable of returning, or recovering, towards said first configuration upon the application of heat alone, have found many applications in diverse fields. Such articles have typically been made from polymeric materials, especially cross-linked polymers, and have been described, for example, in U.S. Pat. Nos. 2,027,962 (Currie) and 3,086,242 (Cook et al).
Quite recently, it has been discovered that such articles can be made from certain metals, sometimes called "memory metals" or "memory alloys". These metals exhibit changes in strength and configurational characteristics on passing through a transition temperature, in most cases the transition temperature between the martensitic and austenitic states, and can be used to make heat recoverable articles by deforming an article made from them whilst the metal is in its martensitic, low temperature, state. The article will retain its deformed configuration until it is warmed above the transition temperature to the austenitic state when it will recover towards its original configuration. The deformation used to place the material in the heat-unstable configuration is commonly referred to as thermally recoverable plastic deformation and can also, in certain cases, be imparted by introducing strains into the article above the transition temperature, whereupon the article assumes the deformed configuration on cooling through the transition temperature. It should be understood that the transition temperature may be a temperature range and that, as hysteresis usually occurs, the precise temperature at which transition occurs may depend on whether the temperature is rising or falling. Furthermore, the transition temperature is a function of other parameters, including the stress applied to the material, the temperature rising with increasing stress.
Amongst such memory metals there may especially be mentioned various alloys of titanium and nickel which are described, for example, in U.S. Pat. Nos. 3,174,851, 3,351,463, 3,753,700, 3,750,552, British Pat. Nos. 1,327,441 and 1,327,442 and NASA Publication SP 5110, "55-Nitinol-The Alloy with a Memory, etc." (U.S. Government Printing Office, Washington, D.C. 1972), the disclosures of which are incorporated herein by reference. The property of heat recoverability has not, however, been solely confined to such titanium-nickel alloys. Thus, for example, various beta-brass alloys have been demonstrated to exhibit this property in, e.g. N. Hakanishi et al, Scripta Metallurgica 5, 433-440 (Pergamon Press 1971) and such materials may be doped to lower their transition temperatures to cryogenic regimes by known techniques. Similarly, 304 stainless steels have been shown to enjoy such characteristics, E. Enami et al, id at pp. 663-68. These disclosures are similarly incorporated herein by reference.
British Patent Specification Nos. 1,327,441 and 1,327,442 describe how this property of heat recoverability can be used to fabricate compression sleeves (i.e. tubular articles in which the forces of heat recovery are directed radially inwardly) useful in joining cylindrical substrates such as hydraulic lines and other conduitry employed in aerospace applications. In the fabrication of these and other such recoverable couplings, the couplings are cooled below their transition temperature, for example by immersion in liquid nitrogen, and are diametrally expanded by forcing a mandrel through them, the mandrel tapering outwardly to a transverse dimension greater than the original internal diameter of the coupling.
In monolithic heat recoverable metallic couplings of this type, the interior surface of the coupling is machined prior to diametral expansion in order to provide circumferential teeth which "bite" into or otherwise deform the surface of a substrate about which the coupling is subsequently heat recovered, enhancing the ability of the resulting article to resist tensile stress and, in particular instances, to achieve a gas-tight interface between coupling and substrate (as used herein, the term "gas-tight" signifies the ability of a coupling-substrate interface as is found in a pipe joint to pass not more than one bubble per minute over a five minute period when an article pressurized with nitrogen at 3000 psig is immersed in water).
Various problems arise from the provision of such teeth on the interior surface of monolithic heat recoverable metallic couplings. First of all, the teeth are subjected to enormous local pressures during mandrel expansion, with consequent damage to the teeth frequently sufficiently severe as (a) to impair the ability of the coupling to form a gas-tight joint, (b) to reduce the tensile strength of the joint and (c) to lower the amount of pressure the joint is capable of withstanding. Secondly, many metallic materials susceptible to the impartation of heat recoverability are difficult to machine.
Other problems have arisen from the monolithic nature of couplings heretofore employed. For one, such couplings tend to recover prematurely when placed over a warm substrate, requiring the use of special chilling tools to abate recovery prior to proper positioning of the coupling on the substrate. Furthermore, rigid quality control procedures have been required to ensure that, prior to application, any lubricant deposited on the interior surface of the coupling to aid mandrel expansion has been removed. Finally, the somewhat limited range of materials susceptible to the impartation of heat recoverability has, in certain instances, prevented the optimum pairing of compression sleeve and substrate materials from mechanical, chemical and electrical standpoints such, for example, as corrosion compatibility, thermal expansion properties, creep resistance, sealability, compatibility of elastic modulus and high temperature strength.